In some prior art FM radio receivers in home-use audio systems, a pulse count demodulating circuit is employed as its demodulating means.
The pulse count demodulating circuit is configured to change an FM wave to a rectangular wave signal, with its upper and lower peaks removed by a limitter, and to produce a rectangular wave which is equal in the rising phase to the former rectangular wave and always uniform in the width from the rising and dropping, so as to subsequently integrate the rectangular wave by an integrating circuit and produce an audio signal.
The pulse count demodulating circuit is low in distortion and high in signal-to-noise (S/N) ratio when the electric field intensity of the received wave is higher than a predetermined level. For this reason, the use of the pulse count demodulating circuit has been recently increased in FM radio receivers in home-use audio systems.
The pulse count demodulating circuit, however, cannot effect its demodulating function and rather produces a large noise when the received electric field intensity is weak. In some home-use audio systems, hard muting is employed to prevent noises. However, hard muting is not suitable in automobile radio receivers including the pulse count demodulating circuit, because due to frequent changes in the received electric field intensity in a car radio receiver, hard muting will be often effected every time when the received electrical field intensity drops below a predetermined level, and will hardly provide sufficiently continuous reproduced audio sound.
This is the reason why the prior art could not use the pulse count demodulating circuit in automobile radio receivers.